Radio transmitters receive a baseband signal, shift the baseband signal up to a higher frequency, and transmit the resulting radio frequency signal via an antenna. Radio receivers shift a received radio frequency signal down to a baseband signal and process the baseband signal to access the information content of the signal. Sometimes radio transmitters and radio receivers are combined in a device referred to as a radio transceiver. In transmitting radio frequency signals it is desirable to restrict transmissions within an allowed frequency spectrum to avoid interfering with other radio communications. Local radio authorities, for example the Federal Communications Commission (FCC) in the United States, establish regulations pertaining to radio transmissions.